Image printing devices are configured to convert electronic information corresponding to an image and print the image on a printing medium. Nonlimiting examples of printing devices include laser printers, ink jet printers, copy machines, scanners and photographic processing machines. For convenience, the printing medium is referred to as “paper” herein. Also, the term “ink” is referred to herein as the material that is applied to the printing medium. For example, a laser printer uses a dry toner and an ink jet printer uses a liquid ink. Ink may be black and/or colored.
Image data is determined and communicated to the printing unit. The electronic image data corresponding to the image is rendered into a plurality of print lines, or raster lines, for printing by a printing device. Thus, each print or raster line corresponds to the width of the paper, and the number of print or raster lines corresponds to the length of the paper. Thus, ink is applied to the paper according to the received print or raster line information. Accordingly, a paper transport unit advances the paper across the printing unit in a step-wise fashion, such that the image is printed according to the determined print lines. The direction of paper travel is referred to herein as the “print direction” for convenience. Also, the orientation of a raster line, typically perpendicular to the print direction, is referred to herein as the “scan direction” for convenience.
Typically, printing devices employ some form of mechanical apparatus to either move the paper over the printing region of the printing unit, and/or to move portions of the printing unit over the paper. For example, a paper transport unit advances a sheet of paper in the print direction using a system of rollers and/or belts.
Furthermore, a portion of the printing unit that is designed to apply ink to the paper may be moved in proximity to the paper medium along the scan direction as a line of print (corresponding to a print line) is applied to the paper. For example, a laser printer employs a charged drum that is spinning such that particles of dry toner are applied to the paper. Thus, a mechanical drive system is used to rotate the drum. Furthermore, a system of moving mirrors may by employed to direct one or more laser beams to apply a charge corresponding to a raster line to the rotating drum.
Another example is the ink jet printer employing an ink cartridge having an ink nozzle in a printhead. The ink nozzle applies liquid ink to the paper. The printhead typically slides along a track or bar oriented perpendicular to the print direction, thereby moving the printhead along the scan direction. As the nozzle is moved along the width of the paper, ink is applied from the nozzle to produce a printed image according to the determined print lines. Thus, a mechanical drive system is used to move the ink nozzle.
Furthermore, a paper transport unit may be comprised of more than one paper transport sub-unit. For example, one paper transport sub-unit in a laser printer may advance the paper over the charged drum. Another paper transport sub-unit may advance the paper, now having applied dry toner, through the fuser unit such that the dry toner is fused to the paper. If the speed that the two sub-units move the paper are not identical, the movement of the paper over the charged drum may change, thereby inducing image distortion.
For proper printing of the image onto the paper, the operation of the mechanical systems must be accurately synchronized with the incoming stream of raster line data corresponding to the image. Such printing systems are carefully designed to ensure proper synchronization of the various mechanical drive systems with the rasterized image data. Accordingly, individual parts must be designed and manufactured to operate within very precise specifications. Component tolerances must necessarily be very small to minimize error in the operation of the mechanical drive system. Because even small errors introduced by individual components may be cumulative with the errors of other components, each individual component in the mechanical drive system must be precisely designed, manufactured and assembled. Realizing such precision is expensive in both design costs, component costs and manufacturing costs.
As the paper is moved along the print direction after a print line has been applied to the paper, the paper must move in a very precise manner for proper alignment of successive print lines. Imprecise paper movement will cause image distortion because print or raster lines may not print as desired on the paper. Due to mechanical image stretch, printed or raster lines may overlap each other, or may be spaced apart from each other. Furthermore, any error in the printing of the print lines or raster lines onto the paper, such as might be caused by an ink jet printer head being moved at an inconsistent rate will cause image distortion. For convenience, distortion in a printed image induced by a mechanical drive system is referred to herein as “mechanical image stretch” or “image stretch.”
Some printing applications require a very high degree of accuracy in the reproduction of the original image on the printed medium. For example, very precise measurements of the printed object may be needed. Or, if color printing is employed, very accurate color reproduction of the original image may be required. Such accuracy exceeds the visual perception of a person. That is, a visually pleasing printed image may not be a sufficiently accurate reproduction of the original image. Furthermore, if image scaling is required (in either the print direction or the scan direction, or both directions), all portions of the image must be scaled as specified.
As an exemplary cause of error in a mechanical system, consider a roller that is not exactly round. The out-of-round portion of the roller will cause the paper speed to vary as the roller drives the paper forward. When the paper is advanced through the printing unit, the speed variation and/or deviation of the paper movement causes a distortion in the printed image because the paper is not advanced through the printing unit as designed. Many other mechanical components may introduce error in the movement of the various mechanical drive systems. Laser printer charged drums may not be exactly round. Gear teeth may be slightly out of tolerance. Servo motors controlling mirror position may not be precisely tuned to its control system. Belts or cables may be loose. One skilled in the art will appreciate that the possible sources for mechanical image stretch are endless, and that the image stretch associated with each individual printing device will be unique.
Some multi-function printing devices include an image capture unit. For example, copy machines and facsimile (FAX) machines are configured to generate image information corresponding to an image. Such image capture units employ mechanical systems which, if not precisely designed, manufactured and installed, will cause image distortion analogous to the above-described mechanical image stretch. Furthermore, a stand-alone image capture appliance, such as a scanner, may employ mechanical systems that may cause undesirable image distortion when the image is generated.